1. Clean Nano Energy Center State Key Laboratory of Metastable Materials Science and Technology Yanshan University Qinhuangdao 066004 P. R. China
2. Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education School of Materials Science and Engineering Xiangtan University Xiangtan Hunan 411105 P. R. China
3. Guilin Electrical Equipment Scientific Research Institute Co. Ltd Guilin 541004 (P. R. China